THE Murray Darling Basin Authority revealed there will be just under 20 gigalitres to trade from upstream to downstream of the Barmah Choke when the new water year kicks in.

The Barmah Choke is a naturally narrow stretch of the Murray River that limits the volume of water that can be delivered.

MDBA executive director Andrew Reynolds said the Choke had a trade restriction to protect delivery to existing entitlement holders and to maintain the river environment.

"While trading water from downstream to upstream of the Choke is always open, trades from upstream to downstream of the Choke can only happen if the same or greater amount has first gone the other way - this is the balance of trade," Mr Reynolds said.

Each new water year, which mirrors the financial year, resets the opening balance for trade across the Choke to zero.

"We then factor in any water saved downstream of the Choke for increased flows in the Snowy River during the previous year," Mr Reynolds said.

"This year, that gives us an expected opening balance of around 19.9 gigalitres, so there'll be some downstream trade opportunities at the outset."

Mr Reynolds said there were no plans to relax the restriction of trade across the Choke.

"River managers will continue to assess the balance of trade through the Choke in planning for the year ahead, by considering factors like storage levels, expected demand, state allocations, and climate forecasts," he said.